This is concerned with an improved method and apparatus for controlling a grinding machine. It is particularly useful with cam-controlled machines used in grinding rotary combustion engines of the Wankel type, such as described in U.S. Pat. No. 3,663,188 to Nils O. Hoglund, which is hereby incorporated by reference. Conventional grinding machines include a worktable for mounting a workpiece to be ground, a grinding wheel, and means for moving the worktable so that the workpiece is moved past the grinding wheel. In cam-controlled grinding machines, the worktable is fixed relative to a cam member so that the workpiece is moved through a path corresponding to the contoured surface of the cam member by rotation of a drive means that engages the contoured surface of the cam.
In grinding the complicated epitrochoid shape of the combustion chamber of a Wankel engine, the relatively thin-walled casting tends to act like a rubber band. Moreover, the thickness of the wall varies because of the arrangement of cooling channels behind the chamber walls; and port openings permit springback and have the effect of an interrupted cut. Since internal and external grinding conditions are present on the same surface, a relatively heavy cut may be made when grinding an external contour, such as the waist section of the chamber, since the grinding wheel contact area is relatively small and the unit grinding pressure is high. Conversely, when grinding an internal contour, such as those at the long ends of the epitrochoid, the grinding wheel contact area becomes relatively large and the unit pressure decreases, making for a smaller cut. As a result, there are continuously varying amounts of workpiece distortion all along the grinding path and coming to accurate sizes under these conditions is a real problem. Even when an in-process gauging system is used for sparkout control, this only indicates that one point along the contour has come to size. Because of springback and distortion, this does not indicate that any other part of the rubber-band-like part has come to size.